Thanks for your question.
When the OD light flashes, this means a code has been more than likely set in the computer, and will need to be pulled out by using a scan tool. Depending on what the code number is, will help in determining what is wrong with the trans. It can be just an electrical concern, or even a major concern inside the trans.
This sounds like a line pressure leak internal to the trans. Possibly the accumulator. Or an electrical concern like shift solenoids or EPC solenoid. Please check all electrical connections to the transmission itself first. Check fuses also for being blown replace as needed.
The best thing I can tell you is to call up a transmission shop like AAMCO. They will more than likely diag by pulling codes from the computer if any and do line pressure test. These code numbers help to diag what part of the trans has failed and then give you a cost to repair and all this diag is no cost to you . FREE, can't beat that.
If they tell you a certain part is bad inside they will probaly recommend a overhaul. This is normal. Because if they have to go in the trans, it is best to over haul while they are in there. However if it is a solenoid or a wire issue, those usually can be repaired by just dropping the pan.
Under the hood under a black cover that has a bunch of relays and fuses.
Under the dash drivers side.
Be sure to make sure both electrical connections to the trans are tight and no moisture in them.
Check the trans fluid level if you haven't already. If it is discolored and smells, you will need to overhaul the trans or replace with reman unit.